This invention relates to a method and apparatus for editing digital signals which are recorded on a record medium and, more particularly, to such a method and apparatus wherein digital signals, such as PCM-encoded audio signals are electronically edited to carry out an insert mode or assemble mode of electronic editing.
In the field of magnetic recording, two types of editing are well-known: physical editing, wherein information recorded on one magnetic tape, such as audio information, and information recorded on another magnetic tape are combined by splicing the two tapes together; and electronic editing wherein information from a separate source is combined electronically with information previously recorded on a magnetic tape. The physical editing technique generally is used when the information recorded on the magnetic tape is relatively low frequency information, such as recorded audio signals.
Generally, electronic editing is used when the information recorded on the magnetic tape is relatively high frequency information, such as video signals. In electronic editing, edit signals, which may be supplied from a suitable source, such as another record medium, a suitable memory device, or the like, is inserted or assembled with original information. For example, original information on one record medium may be rerecorded onto another record medium until a suitable edit point is reached. At that time, the edit information is substituted for the original information, and this substituted edit information is recorded on the second record medium. Subsequently, the original information is recorded once again on the second record medium, resulting in the edit information being "inserted" into the original information. Electronic editing often is used in preparing video tape for video tape recording (VTR) broadcast purposes.
Recently, high quality audio recordings have been made by digitally encoding the audio signals to, for example, pulse code modulation (PCM) format and then recording such PCM-encoded audio signals. For example, in application Ser. No. 771,350, filed Feb. 23, 1977, left and right channel audio signals are sampled, encoded in PCM form, and the PCM signals are recorded on magnetic tape by a conventional VTR device. Various improvements on the digital, or PCM, recording technique have been proposed in order to avoid or minimize errors that might occur due to drop-out, burst error distortion, and the like. These improvements contemplate various types of error correction recording, some examples of which are described in U.S. application Ser. No. 905,894, filed May 15, 1978, and in U.S. application Ser. No. 30,652, filed Apr. 16, 1979.
Although audio information is recorded by the aforementioned digital recording techniques, it is difficult to utilize physical editing, or splicing, with such recorded signals. Even if digitally recorded tape can be spliced successfully, the presence of a splice will introduce errors into one or more of the digital signals, or data words, which represent the audio information. Therefore, there is a definite need for an electronic editing technique in order to edit the digital signals which are recorded on such magnetic tape.
Audio information generally is slowly-varying and, is sampled at a satisfactory sampling rate, a particular digitized sample, or a data word, may be closely approximated by interpolating the information derived from the next preceding and succeeding digitized samples, or data words. Thus, audio information may be recorded as a single track of data words, and even if one data word is distorted or obliterated in that track, the information represented thereby may, nevertheless, be recovered by conventional interpolation techniques. Furthermore, if the data words are in two pairs, and each pair of data words is used to generate an error check word, such as by use of the well-known full adding code, a distorted or obliterated one data word in that pair may be recovered by processing the remaining data with the error check code. However, if both data words are distorted, or if a data word and its associated error check code are distorted, the information represented thereby may be lost, and the audio information which ultimately is recovered may contain undesired interference and distortion. This is a strong possibility when simple switching circuits are used to insert and/or remove digital signals during an edit operation. The use of such switching circuits may result in the recording of switching noise which distorts or obliterates the data words on the record medium.
As an alternative recording technique, the digital words representing the audio signals may be recorded in parallel tracks, with odd-numbered words recorded in one track and even-numbered words recorded in the other track. It is thought that, even if a substantial number of data words in one track are distorted or obliterated, the information represented by such words may, nevertheless, be recovered by, for example, interpolation techniques or other error-correcting techniques from the undistorted data words in the other track. However, even with this recording technique, when edit signals are switched for recording, switching noise will appear concurrently, or in time-alignment, on both tracks. Hence, a data word in one track as well as a related data word in the other track may be distorted, resulting in loss of information and undesired interference in the audio signals which ultimately are reproduced.